Settling velocity of sediment grains, Part 2: Volume/area ratio as descriptor of particle size and shape in sediment hydrodynamics

Particle settling velocity is a fundamental property for sediment hydrodynamics with implications for sedimentology and engineering. Settling velocity depends on fluid properties and the density and dimensions of sediment grains. However, it is not self-evident what length variable should be chosen to represent particle size and shape in settling velocity equations. A companion paper (Part 1) demonstrated that the ellipsoidal model as defined by the nominal diameter and a shape factor are unsuitable for the description of natural sediment particle dimensions. Mathematical derivation from the balance of forces between particle weight and fluid friction reveals that terminal settling velocity should be a function of particle volume and its maximum projection area. In this paper (Part 2) several candidates for a parameter representing particle dimensions in the settling problem are tested, including the diameters of the nominal and equivalent spheres, and the volume/area ratios of the bounding-box ellipsoidal model and the realistic particle model. The latter is obtained from high-resolution micro-computed tomography for 203 irregular skeletal carbonate grains, providing accurate ratios of particle volume over maximum projection area. This ratio predicts settling velocity more accurately than the other tested variables of particle size. Thus, the volume/area ratio captures both particle size and shape, which are usually modelled separately. This study proposes a new framework for accurate hydrodynamic characterisation of sediment particles.